I don't know if plants are using quantum mechanics (which is a theory or a model of nature). That's a bit like saying that black holes are using general relativity to grow or to attract stuff...
Well its a fact that they utilise quantum mechanics for efficiency in the photosynthesis process. If they used a classical process instead then they would not be so efficient. I think what you mean is that there is probably no purpose behind it, and its just an evolutionary trait developed at some point by trial and error. You're probably right.
I dont think the analogy with black holes works. Plants could have evolved differently. Black holes are not biological (as far as we know) and they certainly do not evolve by taking on new characteristics.